This invention relates to a bearing surface for an orthopedic implant which is made of a Cobalt Chrome molybdenum alloy coating deposited on a titanium (Ti) substrate.
In the 1960's to the 1990's, smaller head size metal-on-UHMWPE bearings (22-28 mm) were widely used in orthopedic industry based on Dr. John Chanley's low wear and low friction concepts, but the small size bearing had a high dislocation rate. Between 2000-2010, larger size metal-on-metal bearing (32-44 mm) were used. These significantly decreased the dislocation rate. In recent years, Stryker Corp. introduced dual mobility or bi-polar acetabular cup systems, which combined all benefits of both small bearings' low friction and large bearings' low dislocation rate.
It is desirable to make a new dual mobility cup that has the combined functional features of the current dual mobility acetabular cups, especially having a 3D titanium foam structure for bone ingrowth, while the bearing surface maintains the use of CoCr as a wear resistant surface. Dual mobility acetabular cups are shown in U.S. Pat. Nos. 7,455,694 and 7,833,276.
One solution is using an existing Titanium shell with a thin film Cobalt chromium molybdenum coating (CoCr). These materials have been approved by FDA in biocompatibility. The Titanium shell can be easily machined into any geometry. Physical vapor deposition (PVD) is an approved technology used in the orthopedic area.
In the past CoCr coating have been made on CoCr substrates for orthopedic bearing applications. These coatings had Co, 28% Cr and 6% Mo composition (ASTM F1537, F75) and were made by magnetron sputtering PVD (MS-PVD).
R. Chiesa, C. Piconi, L. Chiusoli, and L. Vandini in “Surface Treatment For Wear Minimization In A New Design Of Total Knee Replacement”, J. Bone Joint Surg Br 2008 Vol. 90-B No. suppl 160 reported magnetron sputtering physical vapor deposition CoCr on cast CoCr knee surface and conducted knee wear simulator test. The purpose of this study was using CoCr coating to minimize the defects of cast CoCr bearing surface, only abstract was published.
Liliana Badita et al., Lucian Capitano, Liliana Laura Badita, Dumitru Catalin Bursuc, “Damage Of The Co—Cr—Mo Femoral Head Of A Total Hip Prosthesis And Its Influence On The Wear Mechanism,” 12th International Conference on Tribology, Kragujevac, Serbia, 11-13 May 2011, p. 267-273, Victor A. Gonzalez-Mora, Michael Hoffmann, Rien Stroosnijder, F. Javier Gil, “The Role Of Hardness And Roughness On The Wear Of Different CoCrMo Counterfaces On UHMWPE For Artificial Joints,” J. Biomed Sci. and Eng., 2011, 4, 651-656, all reported clinical retrieval studies of MS-PVD CoCr coating on CoCr femoral heads against UHMWPE. They found that after testing the CoCr coating had scratching, cracking, peeling and having tribocorrosion. Also the CoCr coated heads had different hardnesses on the CoCr surface in different locations, ranging from lowest 51.2 HRC (about 528 Vickers) in one location to the highest 61.8 HRC (about 720 Vicker) in another. Spherity inspection showed the coated head had an ovoid shape. Post revision inspection indicated that some area of CoCr coating was completely destroyed.
Victor A. Gonzalez-Mora et al., Liliana Laura Badita, Tribologgical Characterization Of Materials used For Femoral Heads Of Hip Prostheses” proceeding of international conference on innovation recent trends and challenges in mechanics, mechanical engineering and new high-tech products development, MECAHIGHTECH'11, vol. 3, 2011 did a pin-on-Disc wear study of UHMWPE-on-MS-PVD CoCr coated rough CoCr coupons. They did not indicate what kind of PVD method was used for CoCr coating, but reported the coating hardness 884±28 Vickers and surface roughness 100 nm. The wear test was conducted using the pin-on-disc method at 1 Mc at a contact stress 3.54 Mpa in Bovine serum water solution. The CoCr coating did not spall at end of test, but MS-PVD CoCr coating showed highest wear rate of UHMWPE, as compared to mass finished cast CoCr and wrought CoCr. The author attributed high wear rate of UHMWPE to the CoCr coating delamination during the wear testing. The delaminated CoCr particles contributed to third body wear. However, the CoCr coating appeared to be either not polished, or poorly polished, Ra=0.1 micron (100 nm) according to atomic force microscopy. This surface roughness doubled the surface roughness of mass finished CoCr 50 nm, therefore the wear rate conclusions were not comparable. All the above studies were preliminary without detailed manufacturing and test details.
Overall, the MS-PVD CoCr coatings performed worse than conventional bulk CoCr as bearing surfaces. One factor may be attributed to the MS-PVD process itself, which is more suitable for flat surfaces such as magnetic recording discs but not for spherical surfaces. Another factor is the low bond strength of the coating to the substrate. MS-PVD process has very low deposition rate. Also, CoCr coating on CoCr substrate does not introduce any new function to the device. It makes no sense from a commercial point of view.
“Bimetal orthopedic devices,” are known which were composed of bimetal structure of CoCr and Ti structure, Daniel E. E. Hayes, Jr., Alfred S. Depress, III, “Bimetal Tibial Components Construct For Knee Joint Prosthesis,” Hayes Medical, Inc., U.S. Pat. No. 7,513,912, Alfred S. Despress, Eugene J. Elwod, Robert R. Aharonov, Peter Ehlers, Knut Andersen, “Implant With Composite Coating,” Hayes Medical Inc., U.S. Pat. No. 6,261,322, Daniel E. E. Hayes, Jr., Alfred S. Depress, III, “Bimetal Acetabular Component Construct For Hip Joint Prostheses,” Hayes Medical Inc., U.S. Pat. No. 6,827,742, H. Ravindranath Shetty, Jack E. Parr, “Method Of Bonding Titanium To A CoCr Based Alloy Substrate In An Orthopedic Implant Device,” U.S. Pat. No. 5,323,954, Daniel E. E. Hayes, Jr., Alfred S. Depress, III, “Bimetal Acetabular Component Construct For Hip Joint Prostheses, Hayes Medical Inc., U.S. Pat. No. 7,850,738. In these publications the main orthopedic device structure was CoCr, which provided both structural and wear resistance against UMPWPE, while a Ti coating was used for better bone ingrowth. The multi-arc PVD method was used to make a Ti coating not a CoCr coating for wear resistance.
Coating Ti on CoCr substrate has been used in the orthopedic industry for decades. However, there are shortcoming such as the stress-shielding of the stiff CoCr construct, the drop in fatigue strength as well as the high weight of the total device, D. M. Brunette, P. Tengvall, M. Textor, P. Thmsen, “Titanium In Medicine,” Spinger Verlag Birlin Heidelberg, pages 283-341, 703-777 (2001).
Another coating used in the past is perpendicular magnetic recording CoCr film, which was invented in the 1980's made by MS-PVD, Kazuo Inoue, Noboru Watanabe, Kazuo Kimura, Eiichiro Imaoka Sagamihara, “Perpendicular Magnetic Recording Medium,” U.S. Pat. No. 4,798,765. The purpose of the technology was to make a columnar structured CoCr thin film (about 0.5 μm) that had grains orientation perpendicular to the magnetic disc surface. This was done so that the film had a maximum recording density and high coercive force in the vertical direction, while the film maintained lowest magnetic remains in the parallel direction. This coating composition is about 84% cobalt and 16% chrome. Because implants are submerged in highly corrosive body fluid at 37° C., this magnetic recording CoCr film is not suitable for orthopedic implant applications.
Yet another coating was CoCr films on titanium for oxidation resistance of a titanium alloy. This use is for high temperature turbine applications, Douglas W. Mckee, “Method For Depositing Chromium Coating For Titanium Oxidation Protection,” U.S. Pat. No. 5,098,540, Krshan, L. Luthra, Douglas W. Mckee, “Coating Systems For Titanium Oxidation Protection,” U.S. Pat. No. 5,077,140, Karel Hajmrle, Anthony P. Chilkowich, “Low Friction Cobalt Based Coatings For Titanium Alloys,” U.S. Pat. No. 5,955,151. The composition of the coating is Cr, CoCrAl, CoNiCr, CoCrAlY-BN. The coating methods are MS-PVD, thermal spray. All these coating compositions are not appropriate for implant applications.
Still another coating is called “thick CoCr coating on titanium alloy by thermal spray.” This technology was described in Stellite® 21 CoCr alloy product brochure (http://www.stellite.co.uk/portals/o/stellite%206%20Final.pdf), but never put into practical use, Frederick F. Buechel, Michael J. Pappas, “Prosthesis With Biologically Inert Wear Resistant Surface,” U.S. Pat. No. 5,861,042 in orthopedic industry. These thermal sprayed coatings obviously had low hardness (470-522 VHN 300 g) and bond strength about 9000-11000 psi (62-76 Mpa). This coating is also unsuitable for use as an orthopedic bearing surface because it has 10 times corrosion rate of as-forged CoCr (ASTM) 1537) according to tests performed by the inventors.
A sixth category of coating is thin ceramic coatings on Ti substrate for orthopedic bearing applications, D. M. Brunette, P. Tengvall, M. Textor, P. Thmsen, “Titanium In Medicine,” Springer Verlag Birlin Heidelberg, pages 283-341, 703-777 (2001), Jason B. Langhorn, Ronald Overholser, Bryan Smith, “Ceramic Coated Orthopedic Implant And Method Of Making Such Implants,” US 2011/0066253. The most popular coating was TiN. The only use is for CrN/ZrN thin film coating on CoCr substrates (http://www.aesculapimplantsystems.com/assets/base/doc/DOC832-ASKneeBrochure.pdf). This mentions that the TiN coated Ti6A14V substrate improved abrasion resistance, but was seen to delaminate after around 600,000 cycles in orthopedic articulating surface. All the ceramic coatings were brittle when on a soft titanium substrate, which caused fatigue failure. On the other hand, if using CoCr alloy substrate, the stiff substrate supports thin ceramic coating well, i.e., low Young's modulus ratio of coating to substrate. This could be the reason why thin ceramic coatings have been commercialized on CoCr substrates.
Another prior art coating is using the CoCr alloy itself. Hexagonal close-packed (HCP) phase is harder than face center cubic (fcc) phase. HCP phase is preferred as a bearing surface to increase scratch resistance, but it is hard to form. The hardness of material is linearly increased with the percentage of HCP phase. The maximum hardness (45 HRC) can be obtained when the HCP phase is close to 95% produced by a long annealing time at high temperature, such as 800° C. for 10 hours (A. De J. Saldivar, et al., “Formation of hcp martensite during the isothermal again of an fcc Co-27 Cr-5Mo-005C orthopedic implant alloy,” Metallurgical and Materials Transaction A, Vol. 30A, 1177-1184 (1999). However, for normal casting, wrought, or forging operation, the majority CoCr stays as the fcc phase with only a minor HCP phase.
Up to now, no commercial orthopedic device has a coating with a percentage of HCP CoCr phase on bearing surface.
It has been unexpectedly found that a CoCr thin film can be produced on a titanium shell for an acetabular cup using multi-arc physical vapor deposition (MA-PVD). The major advantages of MA-PVD are its high deposition efficiency and ability to be used on all geometries, especially for concave acetabular cups.